Backport "Avoid some intermediate Lists" to LTS (#20655)

Backports #18572 to the LTS branch.

PR submitted by the release tooling.
[skip ci]

Author: WojciechMazur

compiler/src/dotty/tools/MainGenericRunner.scala

@@ -148,7 +148,7 @@ object MainGenericRunner {
     case (o @ javaOption(striped)) :: tail =>
       processArgs(tail, settings.withJavaArgs(striped).withScalaArgs(o))
     case (o @ scalaOption(_*)) :: tail =>
-      val remainingArgs = (CommandLineParser.expandArg(o) ++ tail).toList
+      val remainingArgs = CommandLineParser.expandArg(o) ++ tail
       processArgs(remainingArgs, settings)
     case (o @ colorOption(_*)) :: tail =>
       processArgs(tail, settings.withScalaArgs(o))

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -2119,8 +2119,11 @@ class Definitions {
     this.initCtx = ctx
     if (!isInitialized) {
       // force initialization of every symbol that is synthesized or hijacked by the compiler
-      val forced =
-        syntheticCoreClasses ++ syntheticCoreMethods ++ ScalaValueClasses() :+ JavaEnumClass
+      syntheticCoreClasses
+      syntheticCoreMethods
+      ScalaValueClasses()
+      JavaEnumClass
+      // end force initialization
       isInitialized = true
     }
     addSyntheticSymbolsComments

compiler/src/dotty/tools/dotc/transform/CheckUnused.scala

@@ -561,11 +561,19 @@ object CheckUnused:
         else
           Nil
       val warnings =
-        List(sortedImp, sortedLocalDefs, sortedExplicitParams, sortedImplicitParams,
-                  sortedPrivateDefs, sortedPatVars, unsetLocalDefs, unsetPrivateDefs).flatten.sortBy { s =>
-        val pos = s.pos.sourcePos
-        (pos.line, pos.column)
-      }
+        val unsorted =
+          sortedImp :::
+          sortedLocalDefs :::
+          sortedExplicitParams :::
+          sortedImplicitParams :::
+          sortedPrivateDefs :::
+          sortedPatVars :::
+          unsetLocalDefs :::
+          unsetPrivateDefs
+        unsorted.sortBy { s =>
+          val pos = s.pos.sourcePos
+          (pos.line, pos.column)
+        }
       UnusedResult(warnings.toSet)
     end getUnused
     //============================ HELPERS ====================================